1. Field of the Invention
This invention relates generally to the field of magnetic resonance imaging (MRI) utilizing nuclear magnetic resonance (NMR) phenomena. The invention particularly relates to interventional MRI systems and to RF coils especially adapted for interventional procedures conducted during or in conjunction with MRI.
2. Description of the Prior Art
Commercial MRI systems of many different designs are now readily available on the market. Some of these designs use large solenoidal cryogenic superconducting magnets for generation of the static polarizing field B.sub.o. Since the patient is totally enclosed within a long tunnel during MRI on such systems, it is difficult to contemplate significant interventional procedures to be conducted during or in conjunction with MRI. However, other commercially available MRI systems use transverse magnet structures in conjunction with permanent magnets, resistive or superconducting electromagnets for generating magnetic flux B.sub.o between opposing transverse magnet poles). With such transverse magnet MRI systems, open access to the imaging area has become a possibility. One such four-poster (for return flux yokes) transverse magnet structure is depicted in related commonly assigned U.S. Pat. No. 4,829,252-Kaufman (the entirety of which is hereby incorporated by reference). Such systems are commercially available, for example, under the trademark "ACCESS" from Toshiba. Here, improved access to the image volume is already provided by forming apertures in the large transmit RF coil formers otherwise encompassing the image volume (even though all four sides of the four-poster magnet structure are left open by the housing).
A later improvement provides flat pancake-like RF transmit coils so as to leave the image volume totally unobstructed (except for RF receive coils). These flat pancake-like RF transmit coils are described, for example, in co-pending commonly assigned U.S. patent application Ser. No. 08/025,418, filed Mar. 1, 1993, naming McCarten et al as inventors entitled "RF Coil Providing Reduced Obstruction Access to Image Volume in Transverse Magnet MRI System" (the entire content of which is hereby incorporated by reference). Unfortunately, MRI processes typically require higher Q, more closely coupled, RF coils for receiving relatively weak NMR RF signals.
Interventional apparatus and methods for use with transverse magnet MRI systems have already been described in various contexts. For example, see commonly assigned issued U.S. Pat. No. 5,155,435 to Kaufman et al entitled "Method and Apparatus for Performing Interventional Medical Procedures Using MRI Imaging of Interventional Device Superimposed with Ghost Patient Image" and commonly assigned U.S. Pat. No. 5,184,074, issued to Kaufman et al (allowed U.S. patent application Ser. No. 07/650,215, filed Feb. 4, 1991) entitled "Real Time MRI Imaging Inside Gantry Room."
As will be appreciated by those in the art, solenoidal RF coils have found considerable application in transverse magnet MRI systems. In significant part, this is because a transverse MRI apparatus provides the static polarizing field B.sub.o directly between two opposed magnet faces which are typically disposed horizontally and thus the polarizing field B.sub.o is disposed vertically therebetween. Accordingly, to obtain the necessary transverse RF magnetic field for effecting NMR phenomena, a solenoidal RF coil is perhaps a natural choice. At the same time, a solenoidal coil disposed around the image volume presents a substantial obstacle to many interventional procedures.